Effect of phosphorus concentration and method of preparation on the structure of the oxide form of phosphorus-nickel-tungsten/alumina hydrotreating catalysts

Two series of phosphorus containing Ni-W/Al2O3 catalysts were prepared by different preparation methods and varying phosphorus content from 0 to 7.6 wt.% P2O5 The influence of the phosphorus concentration and the preparation procedure on the structure and the dispersion of the compounds formed in the oxide form of the catalysts was studied. Diffuse reflectance spectroscopy (DRS), infrared spectroscopy (LR) and X-ray photoelectron spectroscopy (XPS) were used for the characterization of the catalysts. It was demonstrated that introduction of phosphorus in NiW/Al2O3 catalysts impedes the formation of NiAl2O4 and increases the amount of Ni2+(Oh) ions in the oxide form of the samples. This effect of phosphorus is better expressed when phosphorus is first introduced to alumina, followed by co-impregnation with nickel and tungsten, Surface AlPO4 is formed in the phosphorus containing samples irrespective of the preparation procedure. Phosphorus is distributed as a monolayer up to concentrations of 1.3 P at/nm(2) in both series of catalysts. It was also found that the increased phosphorus content in the samples leads to an increased degree of polymerization of the tungsten species via W-O-W bonds. The presence of phosphorus changes the dispersion of the active components. Evaluation of the average particle size of tungsten species shows that it increases from 10 Angstrom for the phosphorus free sample up to 20-25 Angstrom for the samples with high phosphorus content. The relationship between the structure of the phosphorus promoted NiW/Al2O3 catalysts and their hydrodesulfurization activity is discussed. (C) 1997 Elsevier Science B.V.